Prior to the amniocentesis, women should follow all preparatory recommendations provided by their physician (obstetrician-gynecologist). These may include dietary restrictions and medication changes. The test is typically performed in a physician’s office or outpatient clinic.

During the procedure, the woman lies flat on her back on a table. Initially, an ultrasound is performed to obtain an image of the abdomen. During ultrasound, a wand-like instrument is passed over the outside of the woman’s abdomen. This creates sound waves that are translated to a picture that can be viewed on the monitor. The physician will have a clear picture of the fetus, amniotic fluid, placenta and organs in the maternal abdomen.

Once the abdomen has been cleaned with an iodine solution, the physician uses the ultrasound image as a guide while inserting a thin, hollow needle through the skin and into a pocket of amniotic fluid. Great care is taken to ensure that the needle does not harm the fetus or any nearby organs. About 1 ounce (0.3 deciliters) of fluid is withdrawn. In some cases, additional insertions will be necessary to gain the full amount of fluid required. This is especially true if the mother has a multiple pregnancy.

The mother may experience two moments of mild discomfort: when the needle initially penetrates the skin and when the needle penetrates the uterine muscle. The later pain is likely to feel like cramping, and most women can tolerate the discomfort.

Once the needle has been withdrawn, the physician will use ultrasound to check the fetal heartbeat to make sure it is beating normally. In most cases, the procedure takes just a few minutes to complete.

In rare cases, the sample will not yield the information needed to make an accurate diagnosis. However, this is usually not known until the sample has been sent to the laboratory for analysis. As a result, the only alternative may be to repeat the procedure.